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CVE-2026-33543 is a critical authentication bypass vulnerability in FOSSBilling affecting versions 0.7.2 and earlier. The issue exists in the guest API endpoint /api/guest/staff/create, which is intended to be used only once for initial administrator bootstrap. Due to a flawed administrator-existence guard, the endpoint remains accessible even after an administrator account already exists. Specifically, the protection logic uses is_countable() on a value that returns a Model_Admin object or null rather than a countable type, causing the check to fail to enforce the intended restriction. An unauthenticated remote attacker can invoke the endpoint to create a new administrator account and then immediately authenticate with that account.
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What an attacker gets, and what they’ve been doing with it.
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/api/guest/staff/create at the web server, reverse proxy, or WAF layer, especially from untrusted networks. Review existing administrator accounts for unauthorized additions, remove any unrecognized accounts, rotate administrator credentials, and inspect logs for prior access to the endpoint. Limiting external exposure of the FOSSBilling instance can reduce attack surface until the fixed version is deployed.Patch, then assume compromise.
/api/guest/staff/create. After upgrading, verify that the guest bootstrap endpoint is no longer usable once an administrator account exists and confirm that administrator creation is properly restricted to authorized workflows only.No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
2 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.